Reactivity of 3-benzylidene and 3-ethylidene-2,3-dihydro-2-methyl-1,2-benzothiazin-4-one 1,1-dioxide towards alkylidenephosphoranes

The reactivity of 3-benzylidene- and 3-ethylidene-2,3-dihydro-2-methyl-1,2-benzothiazin-4-one 1,1-dioxide 1a,b towards alkylidenephosphoranes 2 has been investigated. The reaction leads to the formation of 6a,7-dihydrodibenzo[c,e][1,2]thiazine 5,5-dioxide derivatives 3, new heteropolycyclic systems containing the 1,2-benzothiazine 1,1-dioxide skeleton. A Michael addition of the alkylidenephosphoranes γ-carbon to the β-carbon of the α,β-unsaturated carbonyl system of (Z)-1a,b followed by a prototropy and an intramolecular Wittig condensation, explain the formation of products 3. Oxidation of compounds 3 gave the dibenzo[c,e][1,2]thiazine derivatives 4 whereas their reaction with PTSA·H2O gave the (1,1′-biphenyl)-2-sulfonamides 5.